In What Week Does the Baby Start Forming

Your Guide to the First Trimester of Pregnancy

Medically Reviewed by James Greenberg, M.D.

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on October 6, 2021

There's a lot going on in there as your baby begins to develop. Here's more about what to expect, including your first trimester to-do checklist.

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You may not look pregnant yet — but chances are you're feeling it. That's because a flood of pregnancy hormones is prepping your body to play host to your baby for the next nine months — so you could be in line for quite a few aches and pains, from fatigue to flatulence.

While you might be less than thrilled with some of these symptoms, try to remind yourself that these temporary discomforts are part of the incredible process that's happening inside: You're growing a child!

How long is the first trimester?

The first trimester lasts from week 1 through approximately the end of week 13 of pregnancy, or about months 1 to 3.

Not sure what week you are in your pregnancy? The first step to nailing your current week down is to calculate your due date. Keep in mind that your date might change (especially if you have irregular periods), so try to go with the flow.

First trimester symptoms

A lot happens for you in the first trimester. A couple of the most common early symptoms of pregnancy you may experience:

• Morning sickness: Unfortunately it doesn't just strike in the morning — and it typically starts up by about week 6 of pregnancy. Ginger tea or drops might help, as can small but frequent meals. If it's severe, you might want to consider talking to your doctor about medications to treat the symptoms of pregnancy-related nausea.

• Tender breasts: So tender, so tingly and so big! You might be wondering where your old boobs went by about week 6.

• Mood swings: You may (or may not) feel up, then down, then up again by week 7. If you have a history of depression or think it might be more serious, talk to your doctor about getting screened for prenatal depression.

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• Weight gain (or lack thereof): Your baby is still very small — which means all told you only need to gain about three to four pounds in your first trimester. If you're suffering from appetite loss, you might even lose a couple of pounds. That's OK, too — as long as your pregnancy weight gain picks up steam in the second and third trimesters. For now, just focus on eating frequent light meals of any high-density nutritious foods (e.g., avocados, yogurt, bananas, whole grain bread or crackers) when you can stomach it. Feeling extra ravenous? Try to keep a check on your caloric intake during pregnancy: You don't actually need any extra calories during your first trimester (though your intake will go up in the second and third trimesters). If you do end up gaining more than is recommended now, focus on getting back on track over the duration of your pregnancy.

As your pregnancy progresses this trimester, you may experience plenty of other pregnancy symptoms — heartburn, constipation, metallic taste, food aversions and headaches. Hang in there: The second trimester will offer welcome relief!

Keep in mind, too, that every woman is different. So just because your mom or sister reported cramping or frequent urination doesn't guarantee either symptom will happen to you too.

Is it normal to have cramps in your first trimester?

Yes, some abdominal cramping and stomach pain is normal in the first trimester. In the earliest stages of pregnancy — when the embryo attaches to the uterus in a process called implantation — light bleeding or mild cramping can occur.

Gas and bloating — two common causes of cramping — also become fairly common in the first trimester. Plus, as your body shuttles more blood to the uterus, you might start to feel some pressure in this area of the body, especially after you have an orgasm (during which you have normal uterine contractions).

Given all of the changes going on in your body during pregnancy, it's understandable to wonder about what's normal — and what's not. In many cases, that odd twinge is not a cause for concern. However it's also important to understand that your risk for miscarriage is highest during the first trimester.

Here are a few symptoms that do always warrant a call to your doctor:

• Heavy vaginal bleeding

• Severe abdominal pain

• Sudden thirst

• Painful urination

• Fever over 101.5 degrees F, chills and/or backache

• Severe puffiness in the hands/face

• Vision disturbances

If you experience any of the above symptoms, call your doctor's office right away. If you don't get someone on the phone within a few minutes, head to the nearest ER.

How much does a baby grow in the first trimester?

During the first trimester alone your baby starts out as a single fertilized cell (a zygote). After the embryo implants itself in your uterine wall, fetal development takes off, and baby develops into a peach-sized bundle of growing limbs, organs and body systems.

Here are a few of the big highlights happening in this exciting time.

• Baby's bones: By about week 6, baby starts to sprout arms, legs, hands and feet — and fingers and toes around week 10.

• Hair and nails: Skin begins forming between weeks 5 and 8, with hair follicles and nail beds forming around week 11.

• Digestive system: By about week 8, baby's intestines will begin forming, and your baby will have already gone through two sets of kidneys (with the third and final set on its way!).

• Sense of touch: Your baby will develop touch receptors on the face (mostly lips and nose) around week 8. By week 12, the receptors will have sprung up on the genitals, palms and the soles of the feet.

• Eyesight: Optic nerves (which pass info from the eyes to the brain and back) and lenses begin to form by week 4, with the retina beginning to form around week 8.

• Heart: By week 5, the tube that will become your baby's heart begins to beat spontaneously. It will become stronger and more regular — and you'll be able to hear it! — around week 9 or 10 (though sometimes later, depending on your baby's position in your uterus).

• Brain: By about week 8 of pregnancy, your baby's brain will be wiggling those developing limbs.

• Sense of taste: Your baby will have developed taste buds that connect to the brain by about week 8. But because the taste pores are still under construction, baby won't be able to taste the surrounding amniotic fluid (which, by the way, tastes like your most recent meal).

Other major first trimester milestones include the formation of muscles, the production of white blood cells to fight off germs and the development of vocal cords.

Early Signs of Pregnancy

First trimester checklist

Your meals, workouts and daily routines may look a lot different now that you're pregnant. Here are the top to-do's for the first trimester.

• Pop a prenatal vitamin. If you haven't already, start taking a prenatal vitamin immediately — doing so beginning in the first trimester helps ensure that your baby is getting the extra vitamins and minerals needed during pregnancy.

• Choose your practitioner. There are a number of different practitioners you can choose for your pregnancy, from OB/GYN or midwife to family physician. So take time to consider your options and pick the right practitioner for your needs.

• Book your first OB/GYN visit. Your doctor will review your medical history and perform a thorough physical exam. You'll likely undergo a battery of tests, including a Pap smear, urinalysis and blood work to determine your blood type and Rh status, hCG levels and the presence of any infections. You'll likely have an initial ultrasound to confirm a heartbeat, date your pregnancy and be sure things are progressing as they should. You might also be screened for genetic illnesses or diabetes, depending on your family history. While your practitioner will ask lots of questions, be prepared to ask plenty of your own: Now's the time to inquire about the safety of any medications you're currently taking, help for quitting smoking or anything else that's on your mind.

• Consider genetic tests. You'll may have a nuchal translucency (NT) screening (between weeks 10 and 13 of pregnancy) to look for Down syndrome and congenital heart defects. Based on your risks, your practitioner may also recommend noninvasive prenatal testing (NIPT) around week 10, a blood screening that looks for chromosomal abnormalities, and/or invasive but more definitive prenatal tests, such as chorionic villus sampling or amniocentesis. Sometimes, NIPT is enough and your doctor will just do that screening and skip the NT.

• Look into health insurance options. The cost of pregnancy varies based on a number of factors — and it's a whole lot more expensive if you don't have health insurance. So now's the time to sign up for a plan or review the terms of your policy, focusing on your premiums and coinsurance to keep your overall costs as low as possible. The good news: You can still take steps to reduce your prenatal insurance costs.

• Make a budget. Growing your family is an excellent — and necessary — time to reevaluate your monthly expenses. So learn the cost of having a baby, then set up your monthly budget.

• Eat right. Now's the time to cut down on caffeine, as well as to learn which foods to avoid and which to feature in your pregnancy diet so you can stock your kitchen accordingly.

• Carve out time for fitness. There are lots of benefits of exercise during pregnancy for you and baby — which can be good motivation to get your 30 minutes of physical activity most days of the week. Not sure where to start? Try these pregnancy-friendly workouts.

• Have sex, if you feel like it. It's fun and safe for baby — plus it has benefits for both of you.

• Start thinking about baby names. If you don't have a baby name in mind, it's never too early to start tossing around ideas.

• Plan to announce your pregnancy. Think about how and when you want to tell your friends and family the good news, and if and when you'll announce on social media. Most women wait until the end of the first trimester to do so, when the risk of miscarriage is lower. And if you're employed, start thinking about when to tell your boss your pregnant and what to say; do your research in advance to understand your company's maternity leave policies.

What to avoid during the first trimester

You have a lengthy list of to-do's now that you're pregnant — but there are also a few things not to do. Here's what to steer clear of during the first trimester.

• The litter box. Cat feces can harbor a potentially harmful parasite that can cause an infection called toxoplasmosis. In the early stages of pregnancy, toxoplasmosis can cause more serious health risks to the fetus, including eye, hearing and other problems, while later in pregnancy, the complications can be less serious. If you think you've contracted the parasite, ask your doctor for a blood test to find out for sure.

• Hot tubs and saunas. Once you're expecting, you should skip any activities that can raise your core body temperature to over 101 degrees Fahrenheit, including hot tubs, saunas or even a luxurious soak in the bathtub. (Same goes for Bikram, or "hot" yoga, and exercising outdoors in sweltering temps.) You don't have to switch to cold showers, but you should bring a thermometer with you into the tub — make sure the water stays at around 97 to 98 degrees F.

• Alcohol. Thinking of toasting to your good news with a glass of bubbly? Think again. There's no amount of alcohol that's been shown to be safe during pregnancy. Binge drinking — defined as four or more drinks in one sitting — can cause complications such as fetal alcohol syndrome (FAS). You can raise a glass, of course — just make sure there's a mocktail in it.

• Raw or undercooked foods. Now that you're expecting, you'll have to hold off on sushi and medium-rare steaks for the remainder of your pregnancy. Raw and undercooked meat, seafood and eggs can harbor infection-causing bacteria and parasites that aren't good for you or your baby. The good news (if you can call it that): Pregnancy symptoms like nausea and food aversions may dampen your newly-pregnant appetite — so you may not find yourself craving that tuna tartare after all.

• Cigarettes and marijuana use. Lighting up isn't safe during pregnancy. Smoking cigarettes can increase the risk of pregnancy complications like low birth weight and early delivery, as might smoking or ingesting marijuana.

The first trimester ushers in some very welcome news (you're pregnant!) along with a few not-so-welcome symptoms (you can't keep down your breakfast). The good news is that the second trimester offers some much-welcome relief from morning sickness, fatigue and the constant need to go to the bathroom every 15 minutes. Better days are right around the corner!

From the What to Expect editorial team and Heidi Murkoff, author ofWhat to Expect When You're Expecting. What to Expect follows strict reporting guidelines and uses only credible sources, such as peer-reviewed studies, academic research institutions and highly respected health organizations. Learn how we keep our content accurate and up-to-date by reading our medical review and editorial policy.

• What to Expect When You're Expecting, 5th edition, Heidi Murkoff.
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• WhatToExpect.com, Is It Safe to Use Marijuana During Pregnancy?, April 2021.
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• WhatToExpect.com, How Much Does Pregnancy Cost? December 2020.
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In What Week Does the Baby Start Forming

Source: https://www.whattoexpect.com/first-trimester-of-pregnancy.aspx

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